chore(release): prepare 8.x-1.0

Dates the changelog for 8.x-1.0: the Unreleased heading becomes 8.x-1.0 - 2026-10-01, and the four 1.0 entries are tightened to the length of the rc2 ones. No code change. This is the commit the 8.x-1.0 tag goes on.